The drug works not only in patients just starting treatment, but in those who failed earlier treatment, the research found.

The hepatitis C virus can lurk in the body for years, causing liver damage, cirrhosis and even liver failure.

“This is a significant advance in the treatment of hepatitis C,” said Dr. David Bernstein, chief of the division of gastroenterology, hepatology and nutrition at North Shore University Hospital in Manhasset, N.Y., who was not involved in either study.

“We know that if we can get rid of the hepatitis C, we can prevent the progression of [liver] disease,” he said. “This means we can prevent the progression of cirrhosis, we can prevent the development of cancer and also prevent the need for liver transplantation in a large number of people.”

Incivek (telaprevir) was approved by the U.S. Food and Drug Administration in May and is the second drug in a class of drugs called protease inhibitors to be approved to fight hepatitis C. The other drug, called Victrelis (boceprevir), was also approved in May.

The standard treatment for hepatitis C has been a combination of two drugs, pegylated-interferon and ribavirin, which are given for a year. If protease inhibitors such as Incivek are added to the mix, the “viral cure” rate improves and the treatment time is reduced to six months, researchers found.

Both reports were published in the June 23 online edition of the New England Journal of Medicine.

In one study, a Phase 3 trial known as ADVANCE, patients were randomly assigned to either a placebo or the treatment in a double-blind study, which means that neither the patients nor the researchers know who’s getting the drug and who’s getting a sham treatment. This type of study is considered the gold standard for clinical research.

In the ADVANCE trial, 1,088 patients with hepatitis C who had never been treated for the condition were randomly assigned to standard therapy for 48 weeks, or telaprevir combined with standard therapy for eight or for 12 weeks, followed by standard therapy alone for a total treatment time of either 24 or 48 weeks.

The researchers found that 79 percent of those receiving Incivek for the longest period (24 weeks) had a “sustained response,” which basically means their hepatitis C was contained. Among those receiving standard care, 44 percent had a sustained response, the researchers noted.

“We have entered a new era of therapy for hepatitis C, which enables us to cure many more patients than we could before,” said lead researcher Dr. Ira M. Jacobson, from Weill Cornell Medical College in New York City.

Incivek needs to be given along with pegylated-interferon and ribavirin, Jacobson said. The researchers learned early on that Incivek alone reduces the level of the virus, but later the virus can become resistant to the drug, he said.

For the second study, called the REALIZE trial, 663 patients with hepatitis C who had failed standard therapy were divided into three groups. One group received Incivek plus standard therapy, another group was started on pegylated-interferon and ribavirin and then had Incivek added. The third group received standard therapy alone.

Here, the researchers found up to an 88 percent sustained response in patients receiving Incivek, compared with a 24 percent sustained response in the standard treatment group.

“These drugs represent a real milestone in the treatment of this disease,” said lead researcher Dr. Stefan Zeuzem, a professor of medicine at J.W. Goethe University Hospital in Frankfurt, Germany.

“There were very limited treatment options in the past, but now many patients have excellent chances to be cured, even if they already have advanced disease,” he said.

Bernstein noted that in the past, these patients could only be treated with more of the standard therapy for a longer period and the “cure” rate was only 10 percent. “Now you can treat these patients for six months with cure rates approaching 90 percent,” he said. “You are really offering hope to a large number of patients.”

The side effects of the medications include skin rashes, anemia, fatigue, itching, nausea, diarrhea, vomiting and taste changes. Some side effects were serious enough to cause a few participants to drop out, according to the study.

Incivek, made by Vertex Pharmaceuticals Inc., is sold to wholesalers for $49,200 for a four-week course of treatment, said Vertex spokeswomen Nikki Levy.

While both Incivek and Victrelis are important breakthroughs in the treatment of hepatitis C, new drugs with even fewer side effects and perhaps shorter treatment times are in clinical trials, Bernstein said.

Hepatitis C affects almost 4 million Americans, most of whom don’t know they’re infected. Often there are no symptoms, but it is the leading cause of liver transplantation in the United States and is linked to as many as 12,000 deaths a year, the researchers say.